1. Field of the Invention
The present invention relates to a microfilm projection lens system for use in microfilm reader or reader printers and the like to reproduce microfilm images.
2. Description of the Related Art
Microfilm frequently records each frame character and image condition without longitude-latitude standardization. Therefore, during image reproduction (image projection), a prism is disposed between the projection lens and screen, i.e., on the enlargement side of the projection lens, so as to rotate the image and correct longitude-latitude orientation of the reproduced image projected onto a screen.
Conventional projection lens systems, however, must provide an exceptionally large image rotating prism even when said prism is disposed in near proximity to the end face of the enlarging side of the projection lens due to a broadening of the luminous flux which occurs particularly when the angle of field of the projection lens is a wide field angle so as to allow positioning of the entrance pupil at the virtual center of the projection lens system. Further, the image rotating prism is equivalent to a plate parallel to the optical axis arranged so as to be capable of inclination at a 45.degree. angle, the characteristics of which differ depending on position. The aforesaid arrangement has disadvantages inasmuch as the amount of astigmatism produced on the axis increases (astigmatism on the axis is proportional to the length of the bottom face of the prism) as the image rotating prism becomes larger, thereby causing image deterioration, and eventually increasing the overall size of the entire projection optical system including mirrors.
Therefore, various so-called front diaphragm type projection lens systems have been proposed that position the entrance pupil at the edge of the projection lens for maximum focusing of the luminous flux.
In general, front diaphragm type projection lenses produce large curvature of field and astigmatism because the usable field angle is narrow, and coma aberration is difficult to correct in large field angle lens systems.
It is, moreover, extremely difficult to make such a projection lens system in a compact form. Additional very important disadvantages to the aforesaid front diaphragm type of projection lens system are the limited glass material combinations for simultaneous correction of color aberration on the axis and magnification color aberration.